When testing the rotor of an alternator, it is convenient to power it with an excitation current to be able to observe its mechanical behavior when it heats up. The tests are carried out in speed testing pits including metal slideways along which the supports of the rotor bearing slide, as do travelling cranes.
When the speed testing pit is capable of being put under vacuum, it also includes a cylindrical steel sheath to which the slideways and the travelling crane are fixed.
Now, when the rotor is made to rotate with excitation current passing through it, a powerful rotating radial magnetic field is produced which induces currents in all the surrounding metal parts (bearing supports, slideways, travelling crane, and steel sheath if there is one). These currents cause local hot spots, which in turn lead to excessive deformation.
German published patent specification DE-A No. 2 945 144 describes a metal shield which completely encloses the rotor and which is in the form of two semi-cylindrical half shields each made up of semi-cylindrical segments of un-perforated metal sheet which are assembled by bolted flanges, with the two half shields being electrically connected along a vertical join plane and having un-perforated metal face plates at both ends.
In said known device, there are considerable electrical losses in the solid shield which cause heating that can lead to the shield being deformed.
Further, the air imprisoned inside the shield heats up when the rotor rotates, and in turn the rotor is heated also, and in the absence of any ventilation it reaches prohibitive temperatures.
Preferred embodiments of the present invention enable these drawbacks to be avoided.